Add config props unit tests

[onobc]

We currently have primary coverage of both PulsarProperties and PulsarReactiveProperties. However, it would be a good ideal to create a 1st class unit test for each that focuses on the properties logic only.

A good example is the ServerPropertiesTests in SB where they do a direct bind on a map of properties to a config prop class.

This also opens the door to adding defaults tests to make sure that the default of the backing clients are in line w/ the defaults in the config props. See CassandraPropertiesTests for example.

cc: @cbornet

[onobc]

Let's call this one closed @cbornet . Any ones we missed we can add under a separate issue etc.. Thanks for knocking this out!